The field of the invention relates to speed control systems for motor vehicles.
Speed control systems are known which control the engine throttle in response to a difference between vehicle speed and a reference speed. Many of such speed control systems amplify this difference signal by a gain constant.
It is also known to adjust the sensitivity of speed control systems to changes in vehicle speed by adjusting the gain constant. For example, U.S. Pat. No. 3,885,644 issued to Seidler et al increases the gain at high speeds to allegedly provide a more psychologically pleasing operator response.
Gains are also adjusted to address certain deficiencies of a particular speed control system. For example, U.S. Pat. No. 4,803,637 issued to Tada et al discloses a diaphragm coupled to the engine throttle and actuated by engine manifold vacuum. During a preset time after speed control operation is initiated, the system gain is increased to rapidly move the diaphragm in an attempt to reduce speed droop which may otherwise occur during speed control initiation.
The inventors herein have recognized various disadvantages of prior approaches. For example, prior speed control systems do not appear to distinguish between operation while cruising over a relatively flat super highway or encountering a sudden gradient. Accordingly, speed control operation may appear to be hunting on flat surfaces, drooping when encountering uphill gradients and overshooting when traveling downhill.